Planar electron emitter apparatus with improved emission area and method of manufacture

ABSTRACT

The field emission planar electron emitter device is disclosed that has an emitter electrode, an extractor electrode, and a planar emitter emission layer, electrically coupled to the emitter electrode and the extractor electrode. The planar electron emitter is configured to bias electron emission in a central region of the emission layer in preference to an outer region thereof. One structural example that provides this biasing is achieved by fabricating the planar emitter emission layer so that it has an outer perimeter that is thicker in depth than at an interior portion of the planar emitter emission layer, which reduces electron beam emission at the outer perimeter when an electric field is applied between the emitter electrode and the extractor electrode. The electric field draws emission electrons from the surface of the planar emitter emission layer towards the extractor electrode at a higher rate at the interior portion than at the outer perimeter. The planar electron emitter device further includes a focusing electrode electrically coupled to the planar electron emitter.

BACKGROUND OF THE INVENTION

Another application in emitter devices is described in commonlyassigned, co-pending U.S. patent application No. ______ (HP docketnumber 10004242-1), entitled “IMPROVED ELECTRON EMITTER DEVICE FOR DATASTORAGE APPLICATIONS AND METHOD OF MANUFACTURE”, the disclosure of whichis hereby incorporated herein by reference.

The present invention relates generally to emitter devices utilized inultra-high density memory storage systems, and more particularly, thepresent invention relates to an improved solid state emitter thatoptimizes electron emission in a central location to improve focusingaccuracy.

Memory storage systems have made tremendous advancements over the yearsfrom the first use of magnetic tape to magnetic hard drives and nowoptical drives as well as sophisticated fast memory such as S-RAM andD-RAM. A more recent development has utilized field emission electronemitters within an ultra-high density storage device. The field emissionelectron emitters have typically been fabricated in tip-geometry thatemit beams of electrons from the sharp points at the end of the tips.Electron beams are utilized to read or write to a storage medium that islocated proximate the field emitters. An array of field emitters maymatch the array of storage areas within the storage medium or a smallerarray of field emitters may be moved relative to the storage medium toaccess the storage locations on the storage medium.

An example of an ultra-high density storage device utilizing fieldemitter technology is disclosed in U.S. Pat. No. 5,557,596. Each fieldemitter typically generates an electron beam current bound by a storagearea to generate a signal current. Each storage area can be in one of afew different states, and are most typically in a binary state of either1 or 0 represented by a high bit or a low bit. The magnitude of thesignal current generated by the beam current impinging on the storagearea depends on the state of the storage area. Thus, the informationstored in the area can be read by measuring the magnitude of the signalcurrent.

The electron beam may also be utilized to write information into thestorage area. The power of each electron beam can be increased to changethe state of the storage area on which it impinges. By changing thestate of the storage area, a bit of information is stored or erased inthe storage area, depending upon the beam strength.

The speed and accuracy of information stored, retrieved, and accessedgreatly depend upon the efficiency of the field emitters. Further, themanufacturing steps necessary to produce and fabricate field tipemitters is extremely complex. Furthermore, since the storage medium isspaced apart from the field emitters utilized to read or write theinformation thereof, it is necessary to place those elements within aprotective casing under high-vacuum; on the order of 10⁻⁷ Torr or less,in order to protect the delicate surfaces of both the emitter tips andthe memory array from environmental effects. High-vacuums are expensiveand difficult to achieve.

Further, in planar electron emitter technology, when a uniformsemiconductor layer is applied to the emitter electrode, electronemission tends to take place at the edge of an emitter because of fieldconcentration due to extractor electrode geometry. This is not desireddue to significant curvature of electric field lines in that regionwhich causes the beam to become divergent rather than primarilycollimated. It is advantageous to have emission occur primarily in thecenter of an emitter where the extracting field lines are primarilystraight.

What is needed in the field emission electron emitter technology area isa field emission electron emitter that provides a higher efficiency thanthe prior art, that can be made more consistently at a lower cost thanthe prior art, that is more immune to environmental effects as well asthe need for high vacuum environments typically required in the priorart, and that has a greater emission efficiency rate about the centerregion in planar electron emitter devices over that of the prior art.

SUMMARY OF THE INVENTION

According to the present invention, an improved field emission devicefor use within an ultra-high density storage system is disclosed. Thefield emission device is a planar electron emitter that has an emitterelectrode, an extractor electrode, and a planar emitter electronemission layer, electrically coupled to the emitter electrode and theextractor electrode. The planar electron emitter is configured to biaselectron emission in a central region of the emission layer inpreference to an outer region thereof. One example to perform thisbiasing is achieved by fabricating the planar emitter electron emissionlayer so that it has an outer perimeter that is thicker in depth than atan interior portion of the planar emitter emission layer, which reduceselectron beam emission at the outer perimeter when an electric field isapplied between the emitter electrode and the extractor electrode. Theelectric field draws electrons from the surface of the planar emitterelectron emission layer towards the extractor electrode at a higher rateat the interior portion than at the outer perimeter. The planar emitterdevice further includes a focusing electrode electrically coupled to theplanar electron emitter. To achieve the improved electron emission rateat the center region, the planar electron emitter device has a generallyconcave top surface.

In an alternative embodiment, planar emitter emission layer comprises ametal first layer and a semiconductor second layer deposited on themetal first layer. The metal layer may be fabricated from platinum,gold, silver, or a metal semiconductor composite layer while thesemiconductor second layer comprises a wide band-gap semiconductor andis typically very weakly conductive of n-type. Additionally, the planarelectron emitter device according to claim 1 also incorporates adielectric placed between the emitter electrode and the extractingelectrode and another dielectric between the extracting electrode andthe focusing electrode.

A process for fabricating planar electron emitters utilized within anultra-high density memory apparatus comprises the steps of forming anemitter electrode layer, forming an extracting electrode layer, exposingthe emitter electrode layer by removing at least a portion of theextracting electrode layer, and depositing a semiconductor materialabove the emitter electrode in a manner resulting in a controlledthickness gradient extending from a center location of the semiconductormaterial deposited to an outer perimeter of the semiconductor materialdeposited. The process may further comprise, prior to forming theextracting electrode layer, forming a metal layer on the emitterelectrode layer wherein the depositing step places the semiconductormaterial on the metal layer. Additional processing steps consistent withachieving the planar electron emitter device previously described arealso contemplated. These would include fabricating the planar electronemitters so that the semiconductor material deposited above the emitterelectrode forms a concave top surface as well as forming a focuselectrode with necessary insulating dielectric layers.

The planar electron emitter device is intended to be utilized, amongother uses, within a storage apparatus that has a storage medium havinga storage area, the storage area being in one of a plurality of statesto represent the information stored in that storage area. The fieldemitter generates an electron beam current that is utilized to read orwrite the information stored in the storage areas.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the present invention will become apparent tothose skilled in the art from the following description with referenceto the drawings, in which:

FIG. 1 is a cross-sectional view of a planar field emission electronemitter device substrate with polycrystalline layer applied thereto.

FIG. 2 depicts a cross-sectional view of the planar field emissionelectron emitter device according to FIG. 1 with a metal layer beingdeposited on the polycrystalline layer.

FIG. 3 illustrates a cross-sectional diagram of the planar fieldemission electron emitter device according to FIG. 2 wherein aninsulating semiconductor layer is formed over the metal layer to form aSchottky metal-semiconductor barrier.

FIG. 4 depicts a cross-sectional view of the planar field emissionelectron emitter device according to FIG. 3 wherein additionalinsulating and metal layers are formed in accordance with the presentinvention.

FIG. 5 illustrates a cross-sectional view of the completed planar fieldemission electron emitter device with openings formed to expose thesurface of the semiconductor layer in accordance with the presentinvention.

FIG. 6 illustrates a cross-sectional processing diagram of a planarfield emission electron emitter device having a semiconductor layer thathas a varied thickness in accordance with the present invention.

FIGS. 7 and 8 depict cross-sectional processing diagrams of the methodand stages of manufacture of the planar field emission electron emitterdevice of FIG. 6.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

An improved planar field emission electron emitter structure that istypically utilized within an ultra-high density storage device isdisclosed in FIGS. 1-5. The emitter structure 100 utilizes a solid statemechanism to enhance and improve electron emission for use in structuressuch as ultra-high density storage devices, previously disclosed in U.S.Pat. No. 5,557,596, incorporated by reference for all purposes, and infield emission-based display systems such as the type disclosed in U.S.Pat. No. 5,587,628, incorporated by reference for all purposes. Thestructure is also based on the structure described and illustrated in WO00/70638, published Nov. 23, 2000, as well as French patent No.FR9906254.

The solid state mechanism utilizes a thin metal layer placed upon theemitter electrode of the planar field emission electron emitter device.Next, a thin layer of wide band-gap semiconductor material is placedupon the metal layer, which forms a Schottky metal-semiconductorjunction to enhance electron beam formation and emission. Since theformation of the beam of electrons occurs at an interface protected fromenvironment, the emitter structure becomes less sensitive toenvironmental factors such as contamination, and temporal and spatialemitted beam instabilities due to molecular desorption and adsorptioncommonly found in prior art emitter structures lacking the Schottkymetal-semiconductor junction are minimized. Further, the solid-statemechanism lowers the field required for emission thereby reducing drivervoltage requirements that have been an impediment to the use of planaremitter geometries and can also eliminate the need for intrinsically lowmaterial work-function materials required in the prior art.

The barrier further provides a high level of immunity from contaminantsmigrating in strong electric field gradients associated with the emittertip structures. The field gradients are reduced in the direction of theemitting area, thereby reducing the motion of material contaminants.Additionally, in a planar emitter geometry noise is minimized due to acurrent averaging over a larger emitting area than is otherwise providedin tip geometries. This planar emitter geometry is again made possibleby the use of the Schottky metal-semiconductor junction and loweredfields required to emit electrons. The protective barrier also reducesthe need for high vacuum during the finishing stages as contaminantsthat have plagued the prior art are of little effect now. This furtherreduces the cost of manufacture and improves the life span of devicesincorporating the technology of the instant invention.

Each planar field emission electron emitter structure further includesadditional electrodes that are utilized to perform electron extractionfrom the surface of the field emission electron emitter. An extractionelectrode and a focusing electrode typically operate in conjunction withone another to provide the appropriate electric field necessary to firstextract, then focus, or otherwise control, the emitted electron beam.Generally, planar emitter geometries provide a primarily collimated beamwhile tip emitter geometries provide a divergent beam.

The improved planar field emission electron emitter structures arefabricated using well-known semiconductor fabrication techniques such asthose practiced by those skilled in the art. Typically, and for purposesof example only, the described methods and structures are performed onsilicon substrate, but other semiconductor materials may be readilysubstituted, such as using gallium arsenide or germanium in place ofsilicon, or the substrate can be nonconductive as glass or sapphire. Aprocess, with the resulting structure at various stages and atcompletion of the planar field emission electron emitter, is nowpresented in conjunction with FIGS. 1-5.

FIG. 1 illustrates a cross-sectional view of a planar emitter device 100that begins with a substrate 110 upon which the emitters are fabricatedin various layers of semiconductor material, metal material, or oxidelayers according to techniques and procedures well known to thoseskilled in the art. First, an electrode layer of conductive material 112is fabricated on the first substrate 110. The top surface of thesubstrate may be planarized using generally accepted methods such aschemical-mechanical polishing (CMP). The layer 112 is typicallycomprised of metal or doped polycrystalline silicon to serve as thefirst portion of the emitter electrode utilized in the planar fieldemission electron emitter device 100 in accordance with the presentinvention. The conductive layer 112 may be optional in some embodiments.

Next, as illustrated in FIG. 2, a thin layer of metal 114 is depositedupon the surface of electrode layer 112 using conventional metaldeposition techniques known to those skilled in the art. The metal layer114 may be formed from a highly conductive and corrosion resistantmetals (platinum, tungsten, molybdenum, titanium, copper, gold, silver,tantalum, etc. and any alloys or multilayered films thereof) that canbond with a semiconductor to form a Schottky metal-semiconductorbarrier. The conductive layer 112 has a thickness range of 0.1 to 0.5micrometers and metal layer 114 has a thickness ranging from 10 to 100nanometers (nms), with 20 nms being preferred. Alternatively layers 112and 114 can be combined and made of the material constituting layer 114with a thickness assuring appropriate electrical conductivity.

Next, as illustrated in FIG. 3, a second semiconductor layer 116 isdeposited upon metal layer 114. Semiconductor layer 116 is typicallycomprised of a wide band-gap semiconductor material such as titaniumoxide (TiO₂). Other types of wide band gap semiconductor materials wouldalso be suitable and include silicon carbide (SiC), diamond like carbon,SiO₂Al₂O₃, tantalum pentoxide and others.

The metal-semiconductor boundary provides a solid state Schottkymetal-semiconductor barrier. When an electric field is applied,electrons are injected into a field controlled low or negative electronaffinity region within the thin semiconductor layer 116. The emitterdevice 100 utilizes a layer of metal to serve as an electron reservoirand then includes an ultra thin layer of semi-conductor materialcovering the metal layer. The semiconductor material is fabricated toprovide a negative electron affinity surface area that is induced when afield is applied to the structure. For example semiconductor layer 116may be formed from but not limited to materials such as the oxides,nitrides, and oxynitrides of silicon, aluminum, titanium, tantalum,tungsten, hafnium, zirconium, vanadium, niobium, molybdenum, chromium,yttrium, scandium, and combinations thereof. Electrons from the metallayer 114 tunnel through the thin semiconductor layer 116 near itssurface and are emitted from the top of layer 116.

The thickness of insulating semiconductor layer 116 is selected toachieve the negative electron affinity condition upon application of anelectric field. The lower bound on the thickness is determined by theminimum thickness required to create such region. The upper bound on thethickness of the semiconductor layer 116 is determined by the potentialnecessary to cause electron transport in the layer 116. The thicker thesemiconductor layer 116 is, the higher the required potential. As such,the thickness of semiconductor layer 116 has a range of 2 to 8 nms with5 nms being preferred.

After the Schottky metal-semiconductor barrier is formed, additionalconventional processing steps are performed as illustrated in FIG. 4 inaccordance with the present invention. These steps include providingelectrodes proximate the planar field emission electron emitter surfaceson the surface of the emitter 100. Dielectric layers are also formed toprovide separation and insulation from the surface of the emitter aswell between the additional electrode layers. Alternatively Schottkymetal-semiconductor barrier is formed after other structures arecreated.

An insulating dielectric 118 is grown on the surface of emitter 100,using conventional oxide growing and fabrication techniques well knownto those skilled in the art. For example dielectric 118 may be formedfrom, but not limited to materials such as the oxides, nitrides, andoxynitrides of silicon, aluminum, titanium, tantalum, tungsten, hafnium,zirconium, vanadium, niobium, molybdenum, chromium, yttrium, scandium,and combinations thereof. The dielectric 118 may be formed such that theinsulator is conformal with the layer 112. This layer 118 has athickness ranging from 0.5 to 5 micrometers.

Next, a conductive layer 120 is deposited upon an oxide layer 118 usingconventional processing techniques well known to those skilled in theart. The conductive layer 120 may be formed from metal (aluminum,tungsten, molybdenum titanium, copper, gold, silver, tantalum, etc. andany alloys or multilayered films thereof), doped polysilicon, graphite,etc. or combinations of metal and non-metal, e.g. C, films. Conductive120 is typically utilized as an extracting electrode in the emitterstructure 100.

After the formation of the conductive layer 120, an isolating andinsulating layer of dielectric material is applied in layer 122. Layer122 may be identical to layer 118 and fabricated in the same manner orit may be of similar substance to provide a dielectric isolation betweenelectrode metal layer 120 and a subsequent conductive layer 124.

Conductive layer 124 is fabricated on the surface of dielectric layer122 using well-known fabrication techniques similar to that utilized toform layers 114 and 120. Layer 124 may be fabricated out of the samemetal as that used in layers 114 and 120, but it may also be fabricatedout of a different conductive metal typically used by those skilled inthe art. Further, conductive layer 124 serves as a focusing electrode infocusing the emitted electrons from the surface of the emitter duringoperation to the storage medium proximate thereof, for one example.

A final patterning and etching is performed to open holes abovesemi-conductor layer 116 to expose the emitter surface. These techniquesare well known to those skilled in the art and are used to form openingsthrough conductive layers 120 and 124 and to etch back insulatingdielectric layers 118 and 122 in such a way as to provide openings forthe electrons to pass when utilized in their functional design. Theholes typically have a diameter of about 0.1-10 micrometers.

Dielectric layer 122 has a thickness of about one half that of the holediameter and ranges from 0.05 to 5 micrometers. Metal layer 120 has athickness of about 0.05 to 0.3 micrometers. Likewise, conductive layer124 has a thickness range of 0.05 to 0.3 micrometer. Further, althoughit has been depicted that conductive layer 120 serves as the extractingelectrode and conductive layer 124 serves as a focusing electrode, theiroperations may be combined so that they act in tandem to extract andfocus electrons. In another embodiment the wide band gap semiconductorlayer 116 and possibly metal layer 114 are not formed until after theextracting electrodes 120 and 124 and associated dielectric layers 118and 122 are deposited and hole apertures are created. Layers 114 and 116are then deposited through these apertures directly on electrode 112.

It is further contemplated that not a single emitter structure 100 isfabricated at one time but generally an array of such emitter devices100 are fabricated. For example, an array of 100-by-100 emitters 100 maybe made to perform the read and write operations within the ultra-highdensity storage system described earlier. Further, a large array of suchemitter devices may also be utilized in field emission display panels.

Although the emitter structure 100 has been illustrated to have anelectrode layer 112, such a layer is optional such that semiconductorsubstrate 110 is properly doped sufficiently to serve as the emitterelectrode with metal layer 114 deposited thereon. Further, it has beenshown that the emitter is a planar electron emitter with respect to thefabrication techniques and resulting structure depicted in FIGS. 1-5 inaccordance with the present invention, other geometries are alsopossible utilizing the Schottky metal semiconductor barrier approach.The use of the Schottky metal-semiconductor barrier also allows forsmaller geometries to be formed with respect to the focus emitterelectrodes as well as the extracting electrode. The planar electronemitter as shown in FIG. 5 has a focus electrode and extractionelectrode diameter of generally 2 micrometers. It can range from 1 to 10micrometers. The focusing electrode provides the ability to collectelectrons within a small (10 to 50 nm) spot on an anode. Without the useof the focusing electrode, the angle of emission is approximately ±10°for the planar electron emitters.

FIG. 6 illustrates an embodiment of the present invention wherein thesemiconductor layer 216, which is placed upon metal layer 214, isfabricated so that the outer edges 216 a of the semiconductor materialis thicker than the interior portion 216 b of the same. Specifically,the outer edges 216 a have a thickness ranging from 10 to 15 nms whilethe center portion 216 b has a thickness of about 5 nms. The thickersemiconductor material on the outer edges inhibits electron beamemission on the outer perimeter while the thinner semiconductor materialin the central region provides for enhanced electron emission over thatof the outer perimeter. This also greatly improves the emitter emissionefficiency and ability to focus electrons over that of the prior art.

The thicker outer perimeter of the semiconductor material is fabricatedin accordance with the processing steps illustrated in FIGS. 7-8. FIG. 7illustrates a cross section of a second embodiment of an electronemitter according to another aspect of the present invention in aprocess step prior to semiconductor layer deposition.

The second embodiment includes many of the same features describedherein with respect to FIGS. 1-5. At this point the semiconductoremitter electrode 212 as originally described with respect to FIG. 1 isproduced and the metal layer 214 is applied onto electrode layer 212.Next, alternating layers of insulating oxide material and metal layersare fabricated on the surface of metal layer 214. Afterwards, a maskingstep and passivation step are performed in order to open regionsdirectly above the metal layer 214 underneath the oxide and electrodelayers. Thus, as shown in FIG. 7, there is a base substrate layer 210,on which an emitter electrode electron supply layer 212 has been formedand upon that, a metal layer 214 is formed. These steps are consistentwith those previously described in FIGS. 1 and 2. Next, the oxide layer218 has been formed with a portion removed in order to reveal theunderlying metal layer 214. A second metal layer, which serves as anextraction electrode 220, is next formed. Upon extracting electrode 220is formed a second insulating layer of silicon dioxide or itsequivalent. Next, a final metal layer 224 is formed on semiconductorinsulating layer 222. Again, each of these layers is opened insubsequent processing steps to expose a surface of the metal layer 214.Alternatively layers 218 through 224 are deposited sequentially and thehole access to layer 214 is formed through these layers in one step.

Next, as shown in FIG. 7, a parting layer 226 is fabricated on allsurfaces except for the open metal layer 214. The parting layer 226,typically comprised of aluminum or another suitable parting material, isapplied by first rotating the entire substrate about an axisperpendicular to the surface of the substrate and generating acollimated beam of parting material directed at an angle relative to theaxis perpendicular to the surface. The parting layer 226 coats theentire surface, save the surface of the metal layer 214, which isshadowed by the geometry of the access hole.

With the parting layer 226 in place, a somewhat divergent beam ofsemiconductor material 228 is then directed on the substrate, as shownin the cross-sectional illustration of FIG. 8, to grow an insulatingsemiconductor layer, such as titanium dioxide, on the parting layer 226as well as on the bare surface of the metal layer 214 wherein insulatingsemiconductor layer 216 is formed. The beam is directed at an anglerelative to the perpendicular axis and the substrate rotates about theaxis during deposition. The parting layer 226 enables the remainingtitanium dioxide, or the non-necessary semiconductor material to beremoved during a removal step well known to those skilled in the art.Since the semiconductor material is diffused within a somewhat-divergentbeam of application material, the outer perimeter of the layer 216 growsthicker on the outer portion than it does on the center portion becauseof the rotation of the semiconductor substrate and the angle at whichthe beam is applied.

Afterwards, the entire wafer is submerged in parting layer solvent thatremoves the parting layer along with the excess semiconductor materialor titanium dioxide applied to it. Since the semiconductor layer 216bonds physically with the metal layer 214, it is resistant to theparting layer solvent, resulting in the structure shown in FIG. 6. Thisresults in a semiconductor layer 216 that has thicker outer regionsrelative to the center region. The increased thickness on the outerregions inhibits electron emission from these regions while the thinnerthickness of the semiconductor material is more conducive to electronemission, thus increasing the efficiency in the center region thatresults in improved focusing of the beam on a storage medium and thusmore accurate reads and writes during the mass storage read and writeoperations intended for use with the planar field emission electronemitter device of FIGS. 5 and 6.

Other embodiments of the invention will be apparent to those skilled inthe art from a consideration of this specification or practice of theinvention disclosed herein. It is intended that the specificationexamples be considered as exemplary only, with the true scope and spiritof the invention being indicated by the following claims.

It is to be understood that the above-described arrangements are onlyillustrative of the application for the principles of the presentinvention. Numerous modifications and alternative arrangements may bedevised by those skilled in the art without departing from the spiritand scope of the present invention and the appended claims are intendedto cover such modifications and arrangements. Thus, while the presentinvention has been shown in the drawings and fully described above withparticularity and detail in connection with what is presently deemed tobe the most practical and preferred embodiment(s) of the invention, itwill be apparent to those of ordinary skill in the art that numerousmodifications, including, but not limited to, variations in size,materials, shape, form, function and manner of operation, assembly anduse may be made, without departing from the principles and concepts ofthe invention as set forth in the claims.

1-13. (canceled)
 14. A storage apparatus comprising: a storage mediumhaving at least one storage area, the storage area being in one of aplurality of states to represent the information stored in that storagearea; at least one planar electron emitter device to generate anelectron beam current utilized to read and write the information storedin the storage areas, the planar electron emitter device comprising: anemitter electrode; an extractor electrode; and a planar electronemitter, electrically coupled to the emitter electrode and the extractorelectrode, that has an outer perimeter that is thicker in depth than atan interior portion of the planar electron emitter.
 15. The storageapparatus according to claim 14 comprising means of addressing saidelectron beams to storage areas on the storage medium by a motionrelative to one another.
 16. The storage apparatus according to claim 14further comprising means for addressing the electron beams to storageareas on the storage medium by beam steering.
 17. The storage apparatusaccording to claim 14 wherein the planar field emitter further comprisesa focusing electrode electrically coupled to the planar electronemitter.
 18. The storage apparatus according to claim 14 wherein theplanar electron emitter has a generally concave top surface.
 19. Thestorage apparatus according to claim 14 wherein the planar electronemitter comprises a metal first layer and a semiconductor second layerdeposited on the metal first layer.
 20. The storage apparatus accordingto claim 14 further comprising a dielectric placed between the emitterelectrode and the extracting electrode.
 21. The storage apparatusaccording to claim 17 further comprising a second dielectric placedbetween the extracting electrode and the focusing electrode.
 22. Thestorage apparatus according to claim 19 wherein the semiconductor secondlayer comprises a wide band-gap semiconductor. 23-30. (canceled)